Measure the wall where the molding is going to be installed with a measuring tape. Have a partner hold one end of the measuring tape while you measure with the other end of the tape. Write down the measurement.
Mark the measurement from the wall on the plastic molding with a pencil. Pencil marks won't show up on some plastic moldings so you may need to use a marker.
Place the plastic molding on the miter saw. Adjust the molding and saw so they are set up to cut the crown molding to size. Clamp or hold the molding firmly in place and cut the molding. You can also cut the plastic molding with a handsaw.